Carte Du Ciel - The Astrographic Catalogue

The Astrographic Catalogue

For the Astrographic Catalogue, 20 observatories from around the world participated in exposing and measuring more than 22,000 glass plates (see table). Around half of the observatories ordered telescopes from the Henry brothers (Paul and Prosper) in France, with others coming from the factory of Howard Grubb of Dublin. These telescopes were termed normal astrographs with an aperture of around 13 inches (330 mm) and a focal length of 11 feet (3.4 m) designed to create images with a uniform scale on the photographic plate of approximately 60 arcsecs/mm while covering a 2° × 2° field of view. Each observatory was assigned a specific declination zone to photograph. The first such plate was taken in August 1891 at the Vatican Observatory (where the exposures took more than 27 years to complete), and the last in December 1950 at the Uccle Observatory (Brussels), with most observations being made between 1895 and 1920. To compensate for plate defects, each area of the sky was photographed twice, using a two-fold, corner-to-centre overlap pattern, extended at the zone boundaries, such that each observatory's plates would overlap with those of the adjacent zones. The participating observatories agreed to use a standardized telescope so that all plates had a similar scale of approximately 60 arcsec/mm. The measurable areas of the plates were 2.1°×2.1° (13 cm×13 cm), so the overlap pattern consisted of plates that were centred on every degree band in declination, but offset in right ascension by two degrees. Many factors, such as reference catalogue, reduction technique and print formats were left up to the individual institutions. The positional accuracy goal was 0.5 arcsec per image.

Plate measurement was a protracted affair, with measuring done by eye and recorded by hand. The plates were turned over to a large number of semi-skilled "computers" to determine the positions of the stars on each plate. (Before its modern meaning, the word "computer" meant a person who performs calculations). The "computers" would manually measure each star with respect to the dozen or so reference stars within that particular plate, and then perform calculations to determine the star's right ascension and declination. The original goal of 11 mag for the limiting magnitude was generally surpassed, however, with some observatories routinely measuring stars as faint as 13 mag. In total, some 4.6 million stars (8.6 million images) were observed. The brightest stars were over-exposed on the plates, not measured, and therefore missing in the resulting catalogues. The plate measurements (as rectangular coordinates), as well as the formulae to transform them to equatorial coordinates, were published in the original volumes of the Astrographic Catalogue, although the accompanying equatorial coordinates are now of only historical interest. Publication of the measurements proceeded from 1902 to 1964, and resulted in 254 printed volumes of raw data.

For decades the Astrographic Catalogue was largely ignored. The data were difficult to work with because they were available neither in machine-readable form nor in equatorial coordinates. Decades of labour were expended internationally before the project was superseded by modern astronomical techniques. One problem was that the work took much longer than expected. As originally envisaged, the project was meant to have taken only 10 to 15 years. A more serious problem was that while many European astronomers were preoccupied with this project, which required steady, methodical labor rather than creativity, in other parts of the world notably the United States astrophysics was becoming far more important than astrometry. As a result, French astronomy in particular fell behind and lagged for decades.